In recent years, accompanying the remarkable development of portable electronic devices and communication devices and the like, from the viewpoint of cost efficiency and reducing the weight and size of such devices, there is a strong demand for the development of secondary batteries that have a high energy density. Currently, nickel-cadmium batteries, nickel-metal hydride batteries, lithium ion secondary batteries and polymer batteries and the like are available as secondary batteries that have a high energy density. Among these batteries, lithium ion secondary batteries have a particularly longer service life and a particularly higher capacity compared to nickel-cadmium batteries and nickel-metal hydride batteries, and the demand for lithium ion secondary batteries is therefore increasing in the power supply market.
FIG. 1 is a view illustrating a configuration example of a coin-shaped lithium ion secondary battery. As illustrated in FIG. 1, the lithium ion secondary battery includes a positive electrode 1, a negative electrode 2, a separator 3 that is impregnated with an electrolyte, and a gasket 4 that maintains electrical insulation between the positive electrode 1 and the negative electrode 2 and seals the contents within the battery. When charging and discharging is performed, lithium ions move back and forth between the positive electrode 1 and the negative electrode 2 through the electrolyte in the separator 3.
The positive electrode 1 includes a counter electrode case 1a, a counter electrode current collector 1b, and a counter electrode 1c. Lithium cobalt oxide (LiCoO2) or lithium-manganese spinel (LiMn2O4) is mainly used for the counter electrode 1c. The negative electrode 2 includes a working electrode case 2a, a working electrode current collector 2b and a working electrode 2c. A negative electrode material that is used for the working electrode 2c generally includes an active material (negative electrode active material) that can occlude and release lithium ions, a conductive additive and a binder (a binding agent made from resin). These materials are kneaded together with water or an organic solvent to prepare a slurry. The slurry is applied onto the working electrode current collector 2b (for example, a component made from copper foil) and dried to thereby form the working electrode 2c. 
Although a carbon-based material has conventionally been used as the negative electrode active material for lithium ion secondary batteries, in order to increase the capacity of lithium ion secondary batteries, attempts have been made to use materials that occlude and release a larger amount of lithium ions in comparison to a carbon-based material as the negative electrode active material. Silicon oxide may be mentioned as one example of such a material. Silicon oxide is represented by the general formula SiOx (0<x<2), and is obtained, for example, by cooling and depositing silicon monoxide vapor that is generated by heating a mixture of silicon dioxide and silicon. The silicon oxide obtained by such a method includes many amorphous portions.
Silicon oxide occludes and releases a large amount of lithium ions in comparison to a carbon-based material. For example, silicon oxide has a discharge capacity that is approximately five times the discharge capacity of graphite. Therefore, if silicon oxide is used in the working electrode 2c as a negative electrode active material, it is expected that the charge/discharge capacity as a lithium ion secondary battery can be increased. However, it is known that the initial efficiency (ratio of the initial discharge capacity to the initial charge capacity) of the working electrode 2c that uses silicon oxide is as low as about 70%.
To improve the initial efficiency of a lithium ion secondary battery that uses silicon oxide for the working electrode 2c, for example, in Patent Literature 1 that is described below, the use of silicon oxide that has a large proportion of Si relative to O (oxygen) is attempted, while in Patent Literatures 2 and 3 that are described below it is attempted to dope Li into silicon oxide. According to Patent Literature 2, Li is doped into silicon oxide by means of thermal diffusion, while according to Patent Literature 3 Li is doped into silicon oxide by mechanical alloying.
The reason that the initial efficiency of a lithium ion secondary battery improves as a result of doping with Li is as follows. In a case where silicon oxide that has not been doped with Li is used as the negative electrode material, during charging of the lithium ion secondary battery the silicon oxide occludes lithium ions and lithium silicate is formed. Multiple kinds of lithium silicate may possibly be formed. Among such multiple kinds of lithium silicate, some kinds of lithium silicate decompose and release lithium ions, while other kinds do not decompose and do not release lithium ions during discharging of a lithium ion secondary battery.
As a result of the formation of lithium silicate that does not release lithium ions, the initial discharge capacity decreases, that is, an irreversible capacity arises. By doping Li into silicon oxide in advance so that the powder for a negative electrode includes lithium silicate, formation of lithium silicate that does not release lithium ions during charging of a lithium ion secondary battery can be suppressed, and an irreversible capacity can be reduced.